Abstract: The catalytic oxidation of methane over nickel foil is studied. It is shown that, under the oxygen-lean conditions in the regime of a flow-type reactor, nonlinear phenomena can appear in the form of self-sustained oscillations of the reaction rate and the catalyst temperature. To determine the reasons for self-sustained oscillations, X-ray diffraction and mass spectrometry in the operandо mode were used. It was found that the appearance of oscillations in the methane oxidation is due to periodical oxidation–reduction of the surface layer of nickel foil; metallic nickel has a higher catalytic activity than NiO. The oscillations of the catalyst temperature are determined by the occurrence of exothermic and endothermic processes associated with the reduction of nickel oxide and methane oxidation on the surface of metallic nickel.